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Abstract

Single mode fiber (SMF) birefringence effects have been a limiting factor for a variety of Sagnac applications over longer distance SMF links. In this report, we present a new concept of the SMF birefringence compensation in a Sagnac interferometric setup, based on a novel polarization control system. For the destructive interference, our control system guarantees a perfect compensation of both the SMF birefringence and imperfect propagation times matching of the setup’s components. For the stabilization of the constructive interference, we have applied a fiber stretcher and a simple proportional-integral-derivative (PID) controller. The enclosed experimental data of the setup’s visibility confirm validity of our polarization control system. We have also showed that the SMF birefringence model used in a “plug & play” interferometric setup [19], widely cited in the papers on quantum key distribution [11, 12, 13], cannot be applied in SMF Sagnac interferometric setup. However, the SMF birefringence model based on the Kapron equivalence well describes SMF Sagnac.

“Plug & play” interferometric setup. 1 denotes the input, T transmitted output, and R reflected output of the polarization beam splitter, while 1 denotes the input, T the transmitted output in direction 1 → T, and R transmitted output in direction T→R of the circulator. The long arm includes the delay line DL, which causes an additional signal delay compared to the short arm, which does not have such a device.

SMF birefringence compensation in Sagnac interferometer. All the components inside the boxes, marked with dot dashed lines, are polarization maintaining. Couplers CPL1, CPL2, and CPL3 coupling ratios are k=0.5. The delay line’s (DL) length is matched to the stretcher’s (STR) length. The SMF denotes single mode fiber. For the circulator, 1 denotes the input, T the transmitted output in direction 1 → T, and R transmitted output in direction T → R. For the polarization beam splitters PBS1 and PBS2, with the outputs aligned to the slow (horizontal) axis, 1 denotes the input, T transmitted output, and R reflected output. The signal transmitted from the output R into the input 1 shifts its polarization (from the horizontal to the vertical), while there is no such shift for the signal transmitted from the output T into the input 1. DET1, DET2, and DET3 denote photo detectors.

Sagnac visibility measurement result for the total SMF fiber loop length of 50 km. The measurements were carried out during one hour at the 1550 nm laser wavelength with pulse repetition rate of 2 MHz in the burst mode with the burst duty cycle of 20 %. The laser pulse level was set to a classical (non-quantum) signal level. The measurement density is one result/sec since the DAQ card’s reading period of the photo detector DET3 was set to 1 sec.